Millions of metric tons of elemental sulfur are produced each year, primarily as a by-product of natural gas production and petroleum refining, and native sulfur mining industries. Sulfur is also produced as a by-product in coal-fired power plant operations and tar sands development, and in any industrial process that reduces the sulfur level in fuels or effluents for purposes of complying with air quality standards.
In some cases, the sulfur content of naturally occurring hydrocarbons may be as high as 15 vol % or even higher. The presence of sulfur compounds in hydrocarbons is typically highly undesirable, because sulfur compounds are usually extremely harmful, even lethal, to breathe. Moreover, sulfur compounds can be extremely corrosive.
Sulfur compounds recovered from extracted hydrocarbons may take many forms. In some cases, the recovered sulfur compounds are already in the form of elemental sulfur, while in other cases, the sulfur compounds are converted to elemental sulfur for disposal or delivery. In still other cases, the sulfur compounds may be converted to other useful sulfur-based compounds such as sulfuric acid by a WSA Process unit.
Hydrogen sulfide is one example of a common sulfur compound found in naturally-occurring hydrocarbons. Hydrogen sulfide has an extreme acute toxicity, flammability, noxious odor, insidious odor sensory depression, and corrosiveness. In part for these reasons, almost all of the hydrogen sulfide is converted to water and elemental sulfur at or near the site where the hydrogen sulfide is produced.
Because the presence sulfur compounds in extracted hydrocarbon is highly undesirable, hydrocarbon producers usually endeavor to treat produced hydrocarbons to remove sulfur compounds such as hydrogen sulfide. Therefore, produced hydrocarbons are typically processed to remove any sulfur compounds to reduce the sulfur concentration to acceptable levels. Indeed, processing hydrocarbons to remove sulfur compounds is an instrumental part of the hydrocarbon production value chain.
The sulfur compounds recovered from hydrocarbons is either disposed of or transported for end use by others. Typically, the primary sulfur compound recovered from hydrocarbons is elemental sulfur. A continuing challenge in the industry is the transportation or disposal of this elemental sulfur. The refining process which produces elemental sulfur usually produces the elemental sulfur in the form of molten sulfur. Thus, one is faced with the challenge of transporting or disposing of molten sulfur or converting the molten sulfur to some bulk solid sulfur for transportation or disposal. Handling of both forms of elemental sulfur, i.e. molten sulfur and bulk solid sulfur, present significant complications.
Transporting molten sulfur itself without converting it to solid form presents a number of challenges. Proper storage methods are required to ensure the sulfur is not contaminated, that it does not damage equipment (e.g. corrosion, fires), and that it does not harm the environment. Transporting sulfur in molten form requires maintaining its temperature at above approximately 115° C. (−240° F.). While transport over short distances can be done in well insulated containers, over longer distances, a heating system is required to maintain the sulfur in the liquid state. Molten sulfur must be handled and stored within a relatively narrow range of temperatures. Too hot and the sulfur viscosity rises quickly and the sulfur cannot be pumped. Too cold and the sulfur will solidify. Once solidified in a storage vessel, it is difficult to liquefy again due to the low thermal conductivity of solid sulfur. Because molten sulfur is inherently hazardous, systems for transporting molten sulfur involve higher cost to provide the required containment. Moreover, insulation and/or heating mechanisms must be provided during transport to preserve the molten sulfur in its molten state, which necessarily adds additional costs. A tank that has just carried molten sulfur cannot be easily cleaned so that the trailer can carry a different commodity on the return trip or to another destination. The result is that the tank is full on the delivery trip but is empty on the return trip. Larger quantities of molten sulfur may also be transported by rail or by water vessels, but the same transport challenges remain. At the destination, additional heating such as by steam may need to be provided to melt any sulfur that may have solidified during transport. For all of these reasons, handling molten sulfur, either for transportation or for disposal, is beset with a multitude of difficulties and is generally a disfavored method of transporting and/or disposing of sulfur.
Thus, the majority of sulfur around the world is transported as a bulk solid. The sulfur is often stored in the open in huge stockpiles at terminals ready to be loaded onto ships, railcars or truck or at plant sites to be melted and used in the production of sulfuric acid.
Bulk sulfur may be produced from sulfur that has been crushed from larger pieces. Another form of sulfur, slate sulfur, is formed by pouring molten sulfur on a moving belt where it is solidified into a continuous slab with a thickness of 3 to 5 mm. The sulfur begins to break into smaller pieces when it is separated from the belt and when sulfur is discharged from the belt at the head pulley. This process produces irregular shaped pieces with sharp edges.
Granulated sulfur is produced by spray coating sulfur particles to increase their size to produce dense spherical solid granules. Small seed particles of sulfur are introduced at the feed end of a rotating drum. The particles are spray coated with molten sulfur as the particles move down the drum towards the discharge. Each layer of molten sulfur that is applied is cooled to solidification before the next coat is applied. Through repeated application of sulfur layers, a granule size of 1 to 6 mm diameter is produced. Fines are minimal at the production stage and the round shape of the granule resist further degradation to fines.
The WetPrill™ process involves pumping molten sulfur onto a perforated plate. The sulfur flows through the perforations in the form of droplets. The droplets fall into an agitated water bath which solidifies and cools the sulfur into pellets. The pellets are separated from the water in dewatering screens.
While industrial chemicals and commodities can be transported long distances by pipeline, in many cases more economically than by rail or other forms of shipment, pipeline transfer has not been used for sulfur or for only short distances at most. This lack of use is due in part to the high melting point of sulfur, the corrosiveness of sulfur when dissolved in typical solvents or when in contact with air or moisture, and the tendency of sulfur to precipitate from solution. When shipped as a solution or slurry, sulfur tends to deposit on the pipeline walls, resulting in plating, plugging, and line blocking, all of which lead to unreliability, high maintenance, and excessive power consumption.
The storage and disposal of sulfur pose challenges as well, particularly those arising from environmental concerns. Disposal in an environmentally sound yet economical manner is achievable, but at significant expense. Disposal currently consists of converting molten sulfur to solid blocks for above-ground storage, injecting sulfur as acid gas into geologic formations, or oxidizing hydrogen sulfide to sulfur oxides and injecting the sulfur oxides underground for storage. Sulfur disposal as acid gas involves significant injection pressures accompanied by systems mechanical integrity risks. Whereas above ground storage requires a significant environmental footprint and appropriate handling equipment both for the pour and block systems and the recovery of solid sulfur for future sale. Underground fluid injection into existing storage caverns is capital intensive and requires unique geologic conditions.
Thus, conventional methods suffer from a variety of disadvantages, including high cost, inefficiency, and substantial transportation/disposal complications. Accordingly, there is a need in the art for enhanced systems and methods that address one or more disadvantages of the prior art.